<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
    .outer {
        width: 100%;
        height: 200px;
        position: relative;
    }
    .mid {
        position: absolute;
        left: 0;
        top: 0;
        right: 0;
        width: 100%;
        height: 100%;
        background-color: blue;
    }
    .left {
        position: absolute;
        left: 0;
        top: 0;
        width: 100px;
        height: 100%;
        background-color: red;
        /*? 左在中上面*/
        z-index: 999;
    }
    .right {
        position: absolute;
        right: 0;
        top: 0;
        width: 100px;
        height: 100%;
        background-color: red;
        /*? 右在中上面*/
        z-index: 999;
    }

</style>
<body>
<div class="outer">
    <div class="left"></div>
    <div class="mid"></div>
    <div class="right"></div>
</div>
</body>
</html>
